1. Field of the Invention
The present invention relates to an optical system with a wavelength selecting device having high optical performance over the entire image field suitable for video cameras and electronic still cameras equipped with a solid state imaging device.
2. Related Background Art
An imaging device such as a solid state imaging device, which is different from a conventional silver-halide film, has high sensitivity not only visible light but also infrared light having a longer wavelength than visible light. Accordingly, in order to accomplish good color rendition and to remove flare caused by chromatic aberration of long wavelength light, it is necessary to cut off light having a long wavelength of 700 nm or more. So, in conventional video cameras and electronic still cameras using an imaging device such as a solid state imaging device, an infrared-cut filter for cutting off infrared light having a wavelength of 700 nm or more has been arranged at an object side of the imaging device.
With the diffusion of video cameras and electronic still cameras, these cameras are demanded to be compact and light weighted. On the other hand, with getting a pixel of an imaging device such as a solid state imaging device finer and denser year by year, optical performance is demanded to be higher.
Since an aspherical lens can be used freely with recent increase of aspherical surface manufacturing technology, it becomes easy to correct monochromatic aberration such as spherical aberration and coma. So, with increasing optical performance, the optical system can be compact and light weighted by reducing the number of lens elements.
On the other hand, chromatic aberration cannot be satisfactory corrected without increasing the number of lens elements. So, it is not possible to improve optical performance. Accordingly, chromatic aberration becomes an obstacle to improving optical performance of an optical system. In particular, since the light from violet to ultraviolet is liable to be conspicuous, how to suppress this chromatic aberration is the key point to make the optical system have high optical performance. An optical system using the extra-low dispersion glass, fluorite, or the like has been known as a solution. However, since these glass materials have low refractive index, it is liable to incur increase in the thickness of lens and in the number of lens elements. As a result, not only the optical system becomes large, but also it becomes difficult to lower the cost because the material is expensive.
As an alternative way to reduce chromatic aberration in the optical system, it is thought that an ultraviolet-cut filter for removing the most conspicuous range of ultraviolet ray (340 nm to 420 nm) as a chromatic aberration is equipped with the optical system. Wherever it is arranged, the total lens length increases by the amount of the thickness of the filter and an increase in the cost is inevitable.